This invention relates to metallurgy and machine building fields and more specifically to development of methods that improve service life and durability of machine components; to repair of components and reconstitution of their properties; and particularly to gas turbine blades and vanes; and primarily to coatings applied to metal surfaces of aircraft engine compressor blades and vanes.
Frequently, aircraft and helicopters equipped with gas-turbine engines have to operate under conditions of considerable dust content in the air flow and high humidity of sea environment with aggressive elements of corrosive effects. These operation conditions result in abrasion-caused erosion and corrosion of aircraft parts, particularly the compressor blades. Under these conditions, geometry of blades is distorted , operating performances are deteriorated, fuel consumption increases, and engine maintenance and repair expenses grow considerably. The said deteriorating processes can not be efficiently avoided by installation of dust protective devices.
Eroded blades and vanes are generally restored by edge profile polishing or are replaced with new blades and vanes. Such blades and vanes are made of titanium-based alloys or high-alloy steels, which are expensive and difficult to process, so engine repair entails great expense.
U.S. Pat. No. 4,904,542, issued Feb. 27, 1990, reissued under Re.34,173 on Feb. 2, 1993, to Midwest Research Technologies Inc. describes a coating formed of a plurality of alternating layers of metallic and ceramic materials. The two materials selected for the layers have complementary wear resistant characteristics such that one is relatively ductile and the other is relatively hard and brittle. Preferably radio-frequency sputtering is employed to deposit the coating, since it does not produce excessive heating which could negate any prior heat treatment of the substrate onto which the coating is deposited.
Also known are RU Patents No.2,061,090 BI No.15, 1996 and No.2,106,429 BI No. 7, 1998, that describe methods of multi-layer coating deposition on parts and tools, including transition metal coatings. Zirconium is offered as an adhesive bondcoat that is applied to the substrate before the coating; or there is an alternative method of applying metal oxides between the metal layers.
A deposition technique is also known to produce thin films of CNx with implantation of nitrogen ions from plasma. U.S. Pat. No. 5,580,429 issued Dec. 3, 1996, to Northeastern University describes cathodic/anodic vacuum arc sources with a plasma ion implantation deposition system for depositing high quality thin film coatings on substrates. Both cathodic and anodic vacuum arc deposition sources, CAVAD, are used to create a plasma vapor from solid materials composing the cathode and/or anode in the cathodic and/or anodic arc, respectively. Gases, e.g., hydrogen or nitrogen can be in the deposited films by creating a background plasma of the desired gas using either RF energy, thermionic emission, or consequential ionization of the gas passing through the arc or around the substrate. Highly negative pulses are applied to the substrate to extract the ions and provide them with the appropriate energy to interact with the other species in the thin film formation on the substrate to form the desired films. The substrate is bombarded with ionized particles to form carbon nitrides with variable [N]/[C] ratios.
RU Patent No.2,062,818 issued Jun. 27, 1996, describes deposition of metal-containing coatings on large substrates in vacuum. The method includes inert gas ion beam cleaning of the substrate and metal-coating deposition by cathodic sputtering in the inert gas discharge when the substrate is bombarded with the inert gas ion beam that is formed by an accelerator of closed-type drift of electrons at an inert gas ion energy of 50-150 eV. Technically, this method is the closest one to the present invention.
However, the aforesaid U.S. Pat. No. 4,904,542, U.S. Pat. No. 5,580,429 and R.U. 2,062,818, R.U. 2,061,090, and R.U. 2,106,429 do not fully cover the problems of durability and wear resistance, especially as far as aircraft engine blade airfoil surfaces are concerned, which must meet various specific requirements to their wear and corrosion resistance properties and at the same time retain a certain level of their mechanical and, particularly, fatigue characteristics.
Therefore, there is a need to provide improved erosion and corrosion resistance and, as a result, improved reliability and durability of components of various machines, tools and equipment, especially gas turbine engine compressor blades and vanes. That is proposed to be achieved by vacuum plasma technology involving ion implantation.
It is an object of the present invention to provide a technique of coating deposition on metal surfaces, particularly on components of machines, steam and gas turbines, and even more specifically on aircraft engine compressor blades and vanes, that will ensure improved erosion and corrosion resistance and retain the sufficient level of mechanical properties, primarily, fatigue characteristics.
It is a further object to restore the metallic surface of an eroded or corroded metal substrate, particularly the profile surface of a working blade of a gas turbine engine compressor to its original geometric shape and profile parameters.
To achieve the aforementioned objects, a coating is deposited that consists of at least three or four microlayers with certain thickness and compositions. By the term xe2x80x9cmicrolayerxe2x80x9d, in this specification and claims, is meant a layer of pure metal, multiple-component substitution or interstitial metal alloy with non-metal atoms, or interstitial phases based on the said metals, i.e. the metal carbides, nitrides, borides, or complex compounds of the said phases, e.g., carbonitrides, carboborides, etc.
The said coating is produced by means of ion plasma deposition; in the preferred embodiments, the said coating consists of a special microlayer (hereinafter referred to as xe2x80x9csubmicrolayerxe2x80x9d); the said submicrolayer is a rare earth metal, particularly scandium, yttrium or lanthanum and lanthanoids; the said coating also comprises a plurality of microlayers wherein each of said microlayers comprises a material selected from the group consisting of the Group IVA-VIA (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) or alloys thereof, interstitial solid solutions of elements (carbon, nitrogen, boron), nitrides, carbides, or borides of metals, wherein one or more of said microlayers has been subjected to high energy non-metallic (argon, nitrogen, carbon, boron) ion deposition.
The said microlayers of metals or said alloys or metal/non-metal compounds deposited by means of deposition of ions or neutral particles under an appropriate inert gas or non-inert gas, such as nitrogen, methane, acetylene, diborane, should be deposited to a desired thickness, preferably 0.1-10 microns.
The microlayer may be an essentially discrete layer distinct from the adjacent substrate or microlayers; or it may be a mixture therewith. Each of the microlayers may comprise a pure metal or an alloy thereof as prepared, for example, if more than one metallic cathode are simultaneously activated within the chamber or the cathodes are made of alloys. The order of the plurality of microlayers can be selected by opting between corresponding gas atmospheres in the working chamber and by activating the appropriate cathode(s). The number and order of microlayers constituting the fill coating and the inert or non-inert gas ions deposition can be selected depending on the specific requirements determined by the desired performances of machine parts or the whole machine. For example, it is essential that guide blades of the aircraft engine compressor had very hard and wear resistant surfaces, and at the same time fatigue characteristics of the substrate alloy would not play a restrictive role, since such blades are not subjected to high fatigue. On the contrary, working blades of the compressor are very sensitive to fatigue conditions as such blades have to operate under considerable fatigue stresses. Therefore, coatings designed for guide and working blades differ in their thickness and number of microlayers.
The method claimed involves deposition of, at least, three functional microlayers:
1xe2x80x94a damping, corrosion-resistant microlayer of a rare earth metal from the Groups IVA-VIA or a replacement alloy based on said metals, deposited in inert gas atmosphere to the desired thickness, preferably 0.02-5 microns, that provides relaxation of erosion-caused stresses between solid layers and protects from corrosion-aggressive agents of media;
2xe2x80x94a reinforcing microlayer consisting of interstitial solid solutions of nitrogen, boron, carbon in transition metals of the second layer, deposited to the desired thickness, preferably 0.04-10 microns, in a non-inert gas (nitrogen, diborane, methane, or acetylene, respectively, at a partial pressure of said gases 0.05-5xc3x9710xe2x88x921 Pa) atmosphere, that provides a gradual transition to a high strength layer;
3xe2x80x94a wear-resistant, high strength microlayer consisting of interstitial phases such as nitrides, borides, carbides or complex compounds thereof based on said transition metals, deposited to the desired thickness, preferably 0.1-12.5 microns, in corresponding non-inert gas atmospheres at a partial pressure of 0.1-5xc3x9710xe2x88x921 Pa, that provides resistance to erosion effects of abrasive particles.
The deposition of the aforementioned functional microlayers is carried out by activating the appropriate cathode made of a pure metal or a multiple-component alloy, by selecting the necessary partial pressure and composition of the gas atmosphere, and by controlling the appropriate time of deposition as required.
FIG. 1 shows an exemplary microstructure of the claimed coating on an aircraft engine compressor blade of titanium alloy.
Simultaneously, one or more microlayers is subjected to non-metallic (argon, nitrogen, carbon or boron) ion treatment by means of an ion implantor; it is important that such treatment must be carried out directly in the working chamber of the ion-plasma device, simultaneously with or immediately following the deposition process. The ion treatment is carried out with ions at 5xc3x97103-1xc3x97105 eV and radiation dose of 5xc3x971013-1xc3x971018 ion/cm2.
The energy of these implanted ions is considerably higher than the energy of ions formed in the deposition chamber. These ions penetrate deep into the crystal lattice of the deposited metals or interstitial phases and cause changes in the interstitial element concentration, bring about formation of solid solutions and superstructural, non-stoichiometric compounds, and result in submicrostructure and strain modifications. All these result in improved adhesion strength and higher resistance of the coatings to erosion wear. Under the effect of such ion treatment, local temperature peaks may occur followed by rapid cooling of these surface localities, that results in the improvement of strength and tribological properties of the deposited microlayers.
FIG. 2 shows an exemplary X-ray pattern obtained from coatings deposited under various ion implant treatments in the deposition chamber; and FIG. 3 shows the results of investigation by means of the Rutherford back scattering.
It is preferable to use a high energy pulsed ion source in order to reduce risks of overheating and temperature warping of a machine part under ion plasma deposition, which is especially important for aircraft engine compressor blades. Such source produces ions that have the energy high enough for the ions to be implanted into the crystal lattice of deposited phase and to create high-tensile compounds. The following rapid cooling of affected zones prevents the bulk material from overheating, causes the surface substructure to become finer and brings about nanocrystalline or amorphous structures in the surface microlayers.
The ion treatment improves, not only the resistance to erosion and corrosion, but also the endurance limit of machine components, especially under many-cycled fatigue conditions. The improvements are achieved due to the compressing strains generated in the interfacial boundaries and stable defects in the structure caused by the formation of fine precipitates of multiple-component metal/non-metal compounds of variable valence.
However, such complicated processes of multiple-layer coating deposition and ion implanted treatment may induce elevated internal stresses in the surface layers of machine components. In order that the stress distribution be favorable, it is necessary to carry out an additional treatment of the coated parts immediately after the deposition process. After the coated parts are unloaded from the deposition chamber, they are subjected to vibromechanical treatment with micro-pellets.
Therefore, the method claimed includes the following steps:
(a) Preparing the surface for ion-plasma deposition.
(b) Installing cathodes made of metals and alloys to be deposited.
(c) Placing workpieces or substrates into the ion plasma deposition chamber equipped with an ion implantor.
(d) Ion beam cleaning of the surface.
(e) Ion plasma depositing of coating comprising a plurality of microlayers with the required compositions and gas pressures in the deposition chamber.
(f) High-energy ion treatment of one or more microlayers during their deposition or after the coating has been deposited.
(g) Cooling and unloading the workpieces.
(h) Vibro-mechanical treatment according to the preset regime.